Charles Manson, pictured here in a prison mugshot, is to wed next monthGetty/handout.

Cult leader and mass murderer Charles Manson has been issued with a marriage license, and plans to wed a 26-year-old woman who has spent nearly a decade trying to establish his innocence.

Manson, who became one of the most notorious criminals in US history after members of his Manson Family cult killed seven people including pregnant actress Sharon Tate, plans to marry Afton Elaine Burton, a campaigner who has pushed for his release and goes by the name 'Star'.

The Kings County marriage license was issued on 7 November for the 80-year-old Manson and Burton.

Burton moved to Corcoran, California, 10 years ago to be nearer the prison where Manson is incarcerated.

Under the conditions of the license, the couple has until February to tie the knot, and a wedding co-ordinator has been appointed by the prison to organise the ceremony.

Burton told the news agency she and Manson will be married in December.

"Y'all can know that it's true," she said. "It's going to happen."

"I love him," she added. "I'm with him. There's all kinds of things."

Manson and three of his female acolytes were sentenced to death for seven murders and the killing of an unborn infant, but this was commuted to a life sentence in 1972, when California temporarily outlawed the death penalty. Under the terms of his incarceration, Manson is not entitled to conjugal visits.

When asked why she wanted to marry him, Burton told the agency that it would allow her to gain information that she would be able to use in her campaign for his release.

"There's certain things next of kin can do," she said without elaborating.

California Department of Corrections spokeswoman Terry Thornton confirmed to the AP that the license had been transmitted to the prison.

Burton said she expects to be married in a visiting room in the prison, and that the couple will be able to invite 10 guests who are not inmates.

Debra Tate, Sharon Tate's sister, who acts as a spokesperson for the Manson Family crimes, described the marriage as "ludicrous," and asked "What would any young woman in her right mind want with an 80-year-old man?"

When asked about Manson's motives, she said: ""The devil is alive and well."